1. Field of the Invention
The present invention relates to an optical projection apparatus and method. In particular, this invention is related to image projection of luminous screens from electronic devices such as television receivers or computer display terminals.
2. Description of the Related Art
Electronic display technology has advanced rapidly in recent years. The most commonly used display technology is the Cathode Ray Tube (CRT) which is widely adopted in the television and computer terminal display industries. However, other technologies are starting to gain acceptance. Technologies such as the Light Emitting Diodes (LED), Gas Discharge Display and Liquid Crystal Display (LCD) with background light begin to show prominence. Hereinafter, the term "luminous screen" used in this specification and in the appended claims does not restrict itself to any specific display technology. However, for the purpose of a consistent and clear illustration, Cathode Ray Tubes are adopted.
Luminous screens in conventional electronic screen display can only assume a limited size. However, there seems to be an unsatisfied demand for large screen television sets. The benefits of a large screen is multi-fold. A large screen not only avails the audience with the viewing enjoyment of life-like images, it also avoids viewer fatigue from watching a small screen at a long distance for a prolonged time. Other than for regular television program viewing, with the recent proliferation of Video Cassette Recorder (VCR), movies can be enjoyed leisurely at home in an atmosphere similar to a movie-theater.
Enlargements of electronic luminous screen images are also demanded in commercial applications and in education. A product demonstration or an instructional series displayed on a large screen can certainly reach a bigger assembly of audience. Moreover, in classroom instruction, especially in the areas of computer usage or software education, projection of images from the computer display terminal facilitates communications between the instructor and a larger group of students. With the current state of the manufacturing technology, the commonly used Cathode Ray Tubes generally measure no more than approximately 40 inches on the diagonal across the visible portion of the screen. The trend indicates that this seems to be the limit. Beyond this dimension, the cost of manufacturing for building bigger screen tubes will be uneconomical. To satisfy the consumer demand for large screen television sets, giant projection type televisions are marketed by a number of manufacturers. These sets normally include three special projection tubes, each of the tubes projects an image with a primary color, and the images of the three primary colors are combined together on a remote screen. Not only are these sets expensive, they are also very bulky. There exists a need for a projecting system which performs the similar function but is more affordable.
Add-on projection apparatuses that can be adapted to operate with any regular television sets or devices with a luminous screen would be ideal and several of them were invented in the past. Prior art such as U.S. Pat. Nos. 3,900,253 to Astero, Aug. 19, 1975; 4,208,676 to Berwick et al., Jun. 17, 1980; and 4,293,878 to Sanchez, Oct. 6, 1981 all place convex lenses inside enclosures. Generally, the convex lens receives the light from the luminous screen and projects it onto a separate remote screen. Due to the relatively small aperture sizes of the lenses, considerable light energy is lost within the enclosure. Only a small percentage of light energy from the light source is actually transmitted from the luminous screen onto the remote screen. As a consequence, the projected images can only be viewed in a dark room. Moreover, a single set of lenses can only project inverted images. Various methods were used in the prior arts for image re-inverting. U.S. Pat. Nos. 4,208,676 to Berwick et al. Jun. 17, 1980; 3,800,085 to Ambats et al. Mar. 26, 1974; and 4,215,373 to Goldenberg et al. Jul. 29, 1980 depend on a modification of the vertical scan circuitry driving the luminous screen. For a television receiver to be compatible with these devices, internal circuits of the television set need to be altered. Before projection, the image of the television screen is inverted first by turning on a special add-on switch in the altered television set. Other prior art such as U.S. Pat. No. 4,293,878 to Sanchez et al., Oct. 6, 1981 involve an additional intermediate lens in between the luminous screen and the projection lens for image re-inverting. There are several drawbacks with this implementation. Further loss of light results due to the intermediate lens with its limited aperture size. A physically protruding system, which encroaches into the limited projection space is also undesirable. Thus, optical projection apparatus built with lenses of limited aperture sizes possess shortfalls. Part of the reasons for lenses that are normally restrictive in aperture sizes is because ground lenses resulted from the manufacturing process are usually spherical in surface curvature. Too large a lens aperture with a spherical surface curvature and a low f-number would incur all the undesirable optical defects such as spherical aberration, antismatism and coma. To fabricate large aperture lens with all the optical defects corrected would require expensive lens combinations built of materials of different refractive indexes. It is for this reason that apparatuses which use lenses with limited apertures as light processing elements do not utilize the light energy from the luminous screen to an efficient extent, and most of the light is lost inside the enclosure. One further disadvantage associated with prior art projectors that use lenses is that since lenses are adopted for light processing and magnification, chromatic aberration, the focusing of different colors through a lens built of a homogeneous material, is inherent even for lens with a high f-number. The result is a sharp image for one color but blurred for the other colors. To eliminate the undesirable chromatic aberration effect, again, large aperture achromatic lenses have to be used which are expensive.
The aforementioned shortfalls are avoided by projecting apparatuses using mirrors instead of lenses such as shown in U.S. Pat. No. 4,281,353 to Scarborough Jr., Jul. 28, 1981. However, they are large in physical sizes and cumbersome to operate and still do not efficiently utilize the light energy. For example, in Scarborough Jr.,s patent, the system is composed of several separate parts and is space-consuming. Considerable trial and error adjustments have to be made on the mirror stand in order to secure an image on the remote screen. Moreover, the system is applicable to luminous screens with inverted images and there is no mention of how the inverted image is achieved.
It is the object of the present invention to provide an inexpensive and highly efficient apparatus and method of projecting images from a luminous screen.
It is another object of the present invention to provide an apparatus compact in physical size without protruding into the limited projection space.
It is yet another object of the present invention to provide an apparatus which is convenient to use, is substantially maintenance free, which requires no modification of electronic circuitry associated with the luminous screen, and which requires no complicated operating procedures.
It is a further object of the present invention to provide high quality images free of chromatic aberrations and other optical defects such as spherical aberration and coma commonly associated with ordinary lenses.
It is still a further object of the present invention to provide sufficient image brightness and contrast for images to be visible in environments with reasonable ambient light.
It is still yet another object of the present invention to provide an apparatus suitable for luminous screens of different sizes, thus maximizing the convenience of usage and further minimizing manufacturing cost.